1. Field of the Invention
This invention relates to a seal device for a gate valve used for sealing ducts of various vacuum apparatus and vacuum vessels.
2. Description of the Prior Art
As a seal device for a gate valve, there has been one, in which annular members facing the front and rear surfaces of a gate valve member having a through hole are coupled to a valve cage by inner and outer bellows, each of the annular member has an annular recess formed on the side facing the gate valve member and closed by an annular mirror surface finished thin plate secured by welding to an opening thereof, the individual bellows are elongated by fluid under pressure supplied into annular spaces defined by the inner and outer bellows, and the annular mirror surface finished thin plates are strongly urged against the gate valve member by the pressure of the fluid forced from the annular spaces into the annular recesses (as disclosed in Japanese Utility Model Laid-open No. Sho 58-102,879).
This prior art seal device for a gate valve, however, has the following problems. The welded edge portions of the annular mirror surface finished thin plate coupled to the annular member has low mechanical strength and liable to be ruptured when great stress concentration is produced in them with swelling deformation of the annular mirror surface finished thin plate caused by the application of a high pressure to the back side thereof.
In addition, it is likely that the opposite side annular mirror surface finished thin plates fail to be brought into contact with the gate valve member perfectly and when the gate valve is closed. In such a case, the pressures applied to the front and rear surfaces of the gate valve member will be out of balance. Besides, it is likely that the annular mirror surface finished thin plates fail to be urged against the gate valve member with a uniform pressure over the entire circumference. The lack of uniformity of the urging pressure will lead to a partial load on the seal section, thus leading to the rupture of thereof.
Further, when the opposite side annular mirror surface finished thin plates are separated from the gate valve member, they are liable to be brought to positions forming different gaps with respect to the gate valve member. This is thought to be attributable to fluctuations of the precision of fabrication and installation of the bellows.